The Third Generation Partnership Project (3GPP) Release 12 (Rel-12) of the long term evolution (LTE) standard has been extended with support for D2D (also referred to as “sidelink”) features targeting both commercial and public safety applications. Some applications enabled by Rel-12 LTE are device discovery, where devices are able to sense the proximity of another device and associated application by broadcasting and detecting discovery messages that carry device and application identities. Another application consists of direct communication based on physical channels terminated directly between devices.
One of the potential extensions for D2D systems includes support for V2x communication, which includes any combination of direct communication between vehicles Vehicle-to-Vehicle (V2V), Vehicle-to-Infrastructure (V2I) communication, or Vehicle-to-Pedestrians (V2P). V2x communication may take advantage of a Network (NW) infrastructure, when available, but at least basic V2x connectivity should be possible even in case of lack of NW coverage. Providing an LTE-based V2x interface may be economically advantageous because of the LTE economies of scale and it may enable tighter integration between communications with the NW Infrastructure and V2I/V2P/V2V communications, as compared to using a dedicated V2x technology.
V2x communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements sets, e.g., in terms of latency, reliability, capacity, etc. European Telecommunication Standards Institute (ETSI) has defined two types of messages for road safety: Co-operative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM). The CAM message is intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast manner Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. CAM messages also serve as active assistance to safety driving for normal traffic. The availability of a CAM message is indicatively checked, for example, every 100 ms, yielding a maximum detection latency requirement of <=100 ms for most messages. However, the latency requirement for pre-crash sensing warning is typically around 50 ms.
The DENM message is event-triggered, such as by a car braking, and the availability of a DENM message is also checked, for example, every 100 ms. The requirement of maximum latency is <=100 ms. The package size of CAM and DENM messages vary from 100 to 800 bytes, and the typical size is around 300 bytes. The message is supposed to be detected by all vehicles in proximity. The Society of the Automotive Engineers (SAE) also defines a Basic Safety Message (BSM) for Dedicated Short-Range Communications (DSRC) with various messages sizes. According to the importance and urgency of the messages, the BSMs are further classified into different priorities.
Further, there have been discussions within 3GPP regarding reference signals for V2V communications. Major changes are necessary to implement reference signals for V2V communications when compared to LTE legacy because UEs engaged in V2V communications travel at very high speeds (up to 500 km/h relative speed) and may use higher carrier frequencies (up to 6 GHz) than in traditional cellular applications. This leads to larger Doppler spread and Doppler shift that impair the communications.
One proposal under consideration is to transmit DMRS (DeModulation Reference Signals) in all (or at least most) transmitted Orthogonal Frequency Division Multiplexing (OFDM) symbols, but only in a subset of subcarriers, for example, as depicted in FIG. 1 and FIG. 2. In particular, FIG. 1 is a block diagram of a mapping of reference symbols to every OFDM symbol with fixed subcarriers, and FIG. 2 is a block diagram of a mapping of reference symbols to every OFDM symbol with varying subcarriers or subcarrier offset. This manner of mapping DMRS to the subframe is referred to as “2H” (i.e., 2 “horizontal” DMRS per resource block).
Another proposal under consideration is to transmit DMRS in all (or at least most) transmitted OFDM symbols, but only in a subset of subcarriers. For example, as depicted in FIG. 1 and FIG. 2. FIG. 1 illustrates a 1 ms long subframe including fourteen OFDM symbols, one OFDM symbol being the Guard Period (GP) including six subcarriers, and also showing a so-called Automatic Gain Control (AGC) settling. AGC circuits are usually employed in many systems where the amplitude of an incoming signal may vary over a wide dynamic range. The role of an AGC circuit is to provide a relatively constant output amplitude so that circuits following the AGC circuit require less dynamic range. If the signal level changes are much slower than the information rate contained in the signal, then an AGC circuit can be used to provide a signal with a well-defined average level to downstream circuits. In most system applications, the time to adjust the gain in response to an input amplitude change should remain constant, independent of the input amplitude level and hence gain setting of the amplifier. Achieving a constant gain settling time permits the AGC loop's bandwidth to be maximized for fast signal acquisition while maintaining stability overall operating conditions. For both FIG. 1 and FIG. 2, it is not required to transmit the GP OFDM symbol.
The radio communication channel is correlated in time. That is, channel samples taken sufficiently close to each other are similar (in a statistical sense). The properties of time correlation depend on the carrier frequency and the speed of the mobile terminals or User Equipments (UEs) as well as other aspects such as the propagation environment, etc. This correlation is usually exploited by the channel estimation algorithms for example by applying some time-domain filtering.
In the case of synchronous multiuser (or multi UEs) communications, a receiver may receive a linear combination of the reference signals sent by multiple transmitters. Most often, the receiver is interested in estimating the channel from each individual transmitter (rather than the combined channel from all the transmitters). For that purpose, the receiver may make use of the time correlation properties of the channel. One known way doing this is to ensure that the sequences of reference symbols transmitted by the interfering UEs have good cross correlation properties. For example, LTE uses Orthogonal Cover Codes (OCCs) to generate orthogonal sequences and semi-orthogonal base sequences. Semi-orthogonal base sequences are referred to as sequences with low cross correlation properties.
The current sequences used for reference symbols (e.g., DMRS sequences) and the mapping rules for mapping references symbols to physical resource elements fail to provide good performance in multiuser V2V communications, especially if the assumed DMRS mapping is going to be revised by 3GPP into something quite different from, for example, the DMRS mapping so far used for sidelink (or D2D) DMRS. For example, if the density of DMRS REs (Resource Elements) is reduced in the frequency domain, the low cross-correlation properties between signal and interference enabled by the existing DMRS design are reduced and the interference increases.